Page 90 - Basic Electrical Engineering

∈ = ε ε
o r

where ε is the permittivity constant of vacuum and ε is the relative
o
r
permittivity of the dielectric material placed between the two plates.

The value of ε has been calculated experimentally as 8.85 × 10 −12 Farad
o
per meter.

Therefore, the capacitance of a parallel plate capacitor can be expressed as

1.20 ENERGY STORED IN A CAPACITOR

We have known that when a capacitor is switched on to a dc supply, the
charge q can be expressed as q = Cv, where at any instant q is the change, v is
the potential difference across the capacitor plates, and C is the capacitance

of the capacitor.
Potential difference of v volts across the capacitor means v Joules of work

has to be done in transferring 1 Coulomb of change from one plate to the
other. If a small charge dq is transferred then the work done dw can be

expressed as

dw = vdq = Cvdv

The total work done in raising the potential of the capacitor to the supply
voltage of V volt can be expressed as